Conventional spark ignition systems for aircraft turbines use a spark-gap device to control the flow of energy to igniter plugs by way of charge/discharge cycles, which produce sparks at one or more of the plugs for igniting fuel in the combustor of the turbine engine. In such systems, a high voltage power source charges a storage capacitor at a predetermined rate. As the capacitor is charged, the DC voltage at the terminal of the capacitor rises, but the spark-gap device keeps it disconnected from the igniter plug by acting like an open switch. The spark-gap device has a preset breakdown voltage. When the voltage of the capacitor rises to that preset value, the spark-gap device fires (i.e., becomes a closed switch) and discharges the capacitor through an output circuit to the one or more igniter plugs.
The discharge event is relatively instantaneous compared to the time taken to charge the storage capacitor. When the discharge event occurs, the storage capacitor begins a new cycle by recharging at the same predetermined rate as in the previous cycle. This rate is determined by the design of the high-voltage power source. The charge/discharge cycle is thus repetitive and repeats at a rate which is inversely related to the time required to charge the storage capacitor to the breakdown voltage of the spark-gap device. This mode of operation is continuous and free-running--i.e., it is not controlled by a clock or timing device.
Spark-gap devices are very good at switching the high voltage and current of an ignition system from a storage device to an output igniter plug. Furthermore, spark-gap devices are robust and adaptable to the severe environmental extremes experienced by turbine engines in some of their common applications (e.g., aircraft). Moreover, spark-gap devices are well-known two-terminal "passive" devices, meaning they do not require application of external control and/or power signals to function properly. Therefore, they are relatively simple and inexpensive to integrate into a design for an ignition system since they do not require special circuitry.
Until recently, the typical DC ignition system utilized a low frequency blocking oscillator as a high voltage DC-DC converter. This converter depended on the Beta factor of the power transistor to regulate the power processed and has no means for spark rate control over a wide input voltage range. This converter typically would charge the tank capacitor in 500-1000 ms, providing a one (1) to two (2) spark per second output. The power transformer utilized in this DC-DC converter was typically a laminated iron core transformer that was heavy and large. The converter illustrated in FIG. B on page 229 of A. H. Lefebvre's "Gas Turbine Combustion", Hemisphere Publishing Corporation, (1983), is an example of a low frequency blocking oscillator used as a high voltage DC-DC converter.
Newer designs of spark ignition systems employ improved high voltage converters as the power source to charge storage capacitors. These converters are able to charge storage capacitors of ignition systems many times faster than previous converters, by taking advantage of the improvements in magnetic materials and integrated circuits, which enables more power to be processed per unit volume of the transformer, with a resulting reduction in weight and increase in system efficiency. To utilize these new materials, these improved high voltage converters oscillate at a much higher frequency than traditional converters such as the one illustrated in the above-identified Lefebvre reference. Applications of these newer converters have in the past typically been limited to use with discharge circuits that have a controlled rate of discharge. For example, U.S. Pat. No. 5,065,073 to Frus discloses a fast charging, high voltage converter used with a solid-state discharge circuit which depends on an external trigger for initiating a discharge event. By providing an external trigger, the spark rate is controlled. If these new converters are utilized in free-running ignition systems, however, the spark repetition rates are much higher than required to light the turbines and result in accelerated wear of the igniter plugs. Unfortunately, these solid-state discharge circuits are not as robust and rugged as the more conventional free-running discharge circuits using spark-gap devices.